The present invention relates to the use of transmission power regulation techniques in radio communications, and more particularly to a method of and system for regulating power, preferably based on the carrier to interference (C/I) ratio.
To minimize interference between communicators using a radio communication system, the power levels of the radio signals may be regulated. For example, interference between different call connections using the same radio channel in a mobile radiotelephone system might be reduced by regulating the transmission power levels of the mobile stations and base stations. The goal would be to ensure that only the transmission power necessary to maintain satisfactory call quality is used so that the likelihood of interference between calls using the same radio channel would be reduced.
In existing cellular systems, each cell is designed so that a minimal C/I ratio may be obtained in, for example, approximately 90% of the cell under normal traffic conditions. Without transmission power regulation, most of the calls in each cell's interior have a much better C/I than necessary and may cause calls at the peripheries of other cells to use higher transmission power levels in order for those calls to be heard clearly. In other words, the calls in the cell interiors create more interference than necessary without transmission power regulation.
Another advantage of power regulation for cellular telephone systems is that system capacity can be increased by approximately 70% as compared to an unregulated system, assuming the C/I ratio is the same for all calls. Another reason to maintain the power at the lowest possible level is to reduce the energy consumed by the mobile station. Accordingly, the batteries used to power portable mobile stations can have a smaller capacity with the result that the portable mobile stations can be made smaller.
In prior radiotelephone systems, for example as shown in Webb et al. U.S. Pat. No. 4,485,486, power regulation involves the base station measuring the signal strength of transmissions received from the mobile station, comparing the received signal strength with upper and lower thresholds that indicate a desired range for proper reception, and issuing a power adjustment order to the mobile station based on the comparison. The process is repeated at relatively infrequent intervals, typically on the order of once every five seconds, and the mobile's transmission power is controlled only coarsely. This type of control assumes that the disturbance level is more or less constant and tries to keep the received signal comfortably above this level. In practice, however, such power regulation is far from optimal because the disturbance level varies considerably with both time and place.
In the system described in Havel et al. U.S. Pat. No. 4,811,421, the mobile station calculates the transmission power level it should use in order for the strength of the signal received by the base station to be constant. The calculation is based on measurements of the signal strength received by the mobile station that it uses to estimate the path loss to the base station. Thus, the base station does not need to instruct the mobile station to adjust its transmission power. Nevertheless, the regulation is based on maintaining a constant received signal strength, and thus is similar to that disclosed in the patent to Webb et al.
As described in J. Zander, "Optimum Power Control in Cellular Radio Systems", KTH, Report No. TRITA-TTT-9101 (January 1991), for every traffic scenario in a cellular system there exists a maximal C/I ratio which can be obtained by all calls, such that all calls have this same C/I ratio. In a power regulating scheme, it is desirable to regulate power to obtain this maximal C/I value, but in practice it is difficult to determine the value to be targeted. If the value is set too high or too low, the powers of all regulated transmitters will be either increased to maximum levels or decreased to minimum levels, determined by the physical limits of the system.
This "party effect" is one of the problems that have been found in attempting to achieve a constant C/I ratio over an entire cell. For example, if two cells in different cell clusters use the same frequencies and have minimum allowed power levels which the mobiles in the respective cells can aim at and maximum power levels which those mobiles can not transmit above, an unstable system results. Mobiles located in a first cell raise their transmission power levels to overcome the cochannel interference caused by mobiles in the second cell and thus reach the constant C/I ratio. Similarly, the mobiles in the second cell raise their transmission power levels to overcome the now-increased cochannel interference from the mobiles in the first cell. Again, the mobiles in the first cell then adjust their power transmission levels upward. This effect continues until all mobiles in the first and second cells are transmitting at their maximum levels, resulting in no improvement in interference problems, as well as power consumption problems with battery-operated mobiles.
In the system described in Naylor et al. U.S. Pat. No. 4,580,262, the output power level of the transmitter is controlled by the receiver such that the output power is just enough for the link to have sufficient quality. Nevertheless, the quality target is fixed, and thus the system suffers from the "party effect" described above when two call connections use the same radio channel in such a way that they interfere with each other. Such cochannel interference situations are common in cellular radio telephone systems.